Detection of warm SO_2 gas in oxygen-rich AGB stars

Abstract

We report the discovery of the nu_3 band of SO_2 at 7.3 mu m in the ISO/SWS spectra of oxygen-rich AGB stars. The band is clearly detected in three stars, UX Cyg, o Cet and T Cep, and marginally detected in at least four other stars. The band is seen in absorption in UX Cyg, while it is in emission in o Cet. Seven spectra of T Cep taken at different phases show that the feature changes from emission to absorption on a time scale of twice the pulsation period. Using an LTE model, we find that the excitation temperature of SO_2 is typically 600 K, and that in T Cep the molecule occupies a region with dimensions of several stellar radii. The total number of molecules contained in this region is of order 10(47) , which requires a local gas density of at least 10(9) H_2 cm(-3) , and possibly up to 10(11) H_2 cm(-3) depending on the SO_2 abundance. The variation with phase of the T Cep spectra can be explained by photodissociation of SO_2 molecules by soft UV photons. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.